High test hypochlorite is generally obtained by drying calcium hypochlorite dihydrate, and calcium hypochlorite is produced through a diversity of processes. Most of known processes use as raw materials not only lime and chlorine but sodium hydroxide. Typical processes starting with lime, chlorine and sodium hydroxide are disclosed, e.g., in JP-A-50-87994 (The term "JP-A" as used herein means an "unexamined published Japanese patent application"), U.S. Pat. No. 3,251,647, and JP-A-52-134895.
In these processes using sodium hydroxide, calcium hypochlorite dihydrate crystals and sodium chloride crystals precipitate simultaneously, which are fractionated by classification. However, because of insufficient fractionation, both the resulting high test hypochlorite and the by-produced sodium chloride have had insufficient purity.
In order to eliminate the above problem, it seems the most reasonable to make calcium hypochlorite dihydrate by chlorinating lime without use of sodium hydroxide. In this case, the mother liquor comprises calcium chloride as a main component, which is of higher value of use than sodium chloride.
However, because calcium hypochlorite dihydrate crystals have difficulty in growing in the mother liquor comprising calcium chloride, the resulting crystals have the form of silk yarn, which are very difficult to collect by filtration, and the resulting filter cake has a low available chlorine concentration. Addition of water to the mother liquor to decrease the calcium chloride concentration could improve the crystal growth but deteriorates the water balance and the chlorine unit. In addition, the mother liquor becomes less valuable due to the reduced calcium chloride concentration.
JP-A-54-127897 proposes addition of specific prismatic seed crystals to a mother liquor to obtain coarse crystal grains of calcium hypochlorite dihydrate notwithstanding the high calcium chloride concentration of the mother liquor. According to this process, the filterability of the crystals is improved, decomposition during drying is suppressed, and the product has a high available chlorine content. However, the publication has no mention of specific examples of utilization, still less the quality, of the calcium chloride in the mother liquor. Since the mother liquor must contain calcium chlorate, calcium hydroxide, metallic impurities, etc. as well as calcium hypochlorite and calcium chloride, these by-products or impurities should be removed before the calcium chloride can be supplied as a commercial product, but the publication does not show the purification method.
U.S. Pat. Nos. 4,348,371 and 4,348,372 disclose recovery of calcium chloride and calcium chlorate from the mother liquor associated with the manufacture of a calcium hypochlorite product aiming at putting these by-products on the market. The process proposed comprises concentrating the mother liquor to collect calcium chloride crystals, further concentrating the resulting mother liquor to collect calcium chlorate crystals, and recycling the residual mother liquor to the calcium hypochlorite production step. However, the process is disadvantageous in that a large-sized installation and complicated steps are required; that there is a fear of accumulation of impurities, such as metallic compounds, originated in slaked lime as a raw material; and that calcium hypochlorite dihydrate crystals do not grow sufficiently in this recycle system and have poor filterability.
Additionally, since milk of lime from which calcium hypochlorite dihydrate is to be crystallized upon chlorination of lime should have a high concentration, the technique for preparing the milk of lime is bottleneck. In this connection, JP-A-54-127897 shows an example of mixing lime cake and the mother liquor from which calcium hypochlorite dihydrate crystals have been separated. According to the inventors study, it turned out that mere mixing is insufficient for dispersion, tending to form undissolved lumps of lime powder. Such lumps of powder not only make chlorination difficult but may be collected together with the calcium hypochlorite dihydrate crystals to seriously ruin the quality of the product.
That is, the conventional techniques are unsatisfactory from the industrial, economical and ecological considerations; for sodium hydroxide that is theoretically unnecessary is used, calcium chloride that is unavoidably by-produced from chlorination of lime is not made use of, whether or not sodium hydroxide is used in combination, or, if calcium chloride could be made use of, complicated steps and cumbersome operations are involved.